Robotically operated tools are widely used in a variety of applications, including the manufacture of vehicles such as aircraft. A robot typically includes a robotic arm with an end-effector on which the tool is mounted. For the robotic arm to be easily installed and reconfigured according to variations in the manufacturing process and environment, it is desirable for the arm to be light. However, if the tool is required to apply a force to the workpiece, for example during drilling or riveting, then, if this force is reacted through the robotic arm, the arm needs to be sufficiently stiff to sustain the necessary reactive force and to avoid a position slippage during the process. These two requirements are therefore to some extent conflicting.